1. Field of the Invention
The present invention relates generally to methods of manufacturing broaching tools and particularly to a method of manufacturing a broaching tool directly from a preformed blank of heat hardened metal.
2. Description of the Prior Art
Known prior art methods of manufacturing broaching tools involved a two-step operation requiring first an initial rough milling of gullets in a soft blank of metal followed by finish grinding the gullets to form the finished teeth therein once the metal had been heat hardened.
In the initial operation a blank bar of soft unhardened metal was roughly cut to the size requirements of the finished broach. This soft bar of metal was then milled to provide a taper to the bar which gave the finished broach the requisite rise to allow the broaching by the finished tool to be progressively accomplished. With the metal in the unhardened stage, the rough form of the cutting teeth were milled by milling a series of gullets at predetermined spaces in the unhardened metal. Once these operations were completed the metal was put through a heat treating process to harden the metal and provide a keen hardened edge to the finished broaching tool. Heat transfer characteristics of the metal had to be considered during the heat treating process to prevent the creation of cracks in the rough formed cutting teeth of the broach. Once the metal was hardened, the final broach teeth were finished cut in the hardened metal generally using an aluminum oxide cutting wheel.
The reason for this two-stage process in the prior art methods of manufacturing broaching tools was mainly one of speed and economy. It was found that soft metal such as steel may be cut faster by milling. It was also found impossible to mill the final form of the cutting teeth in the soft metal such as steel since distortions occurred during the heat treating, hardening, process. Thus, only the rough form of the teeth were milled in the soft steel followed by finish grinding of the final tooth form using an aluminum oxide wheel on the hardened and rough cut metal bar.
Prior art grinding wheels using aluminum oxide could not cut the finished teeth directly from a hardened blank of metal in the same time as the above two-stage operation since attempts at high speed deep cutting of gullets in the hardened metal blank caused the wheel to quickly deteriorate and the hardened material to develop heat burn spots.
Examples of such known prior art methods of grinding as described may be found in U.S. Pat. No. 3,984,213 which utilizes known grinding techniques using aluminum oxide wheels and in U.S. Pat. No. 2,889,669 which teaches known slow methods of grinding hardened metals.
From the foregoing it may be seen that the prior art two-stage method of manufacturing broaching tools was relatively slow and required a number of operations. Clearly, what was needed was a method of quickly cutting finished broaching teeth to form a broaching tool directly from hardened metal such as steel.